REAR END, DIFFERENTIAL OR PINION

Hi,
I have a 2004 Ford Explorer 4X4 that has an issue with noise coming from rear end. It is cross between a whining and howling sound, but only appears while you are accelerating (usually starts at about 45 MPH)
When you let off the gas, the sound goes away.
Yesterday, Tires Plus replaced the rear end wheel bearings for about $900. The noise is still there and now they told me it might be the differential. And that due to bad bearings, it put stress on the rear end, thus causing the pinion or gears to go bad. After spending $900. I can't aford to have anymore work done. But I am driving to Buffalo from Atlanta and back for the holidays. First question, is it safe to drive this car that far with a bad differential? And second, what is the average cost of having this gear repaired? Third, without hearing this noise, does this sound like a bad pinion to you? And last but not least, is this problem common to 04 Explorers. I am the type of person that takes very good care of my vehicles. Oil every 3000 etc. Thank you, Bill (nervous in Atlanta) I leave this weekend on my trip north.